systems: chapter 8 ion channels

What are ion channels

they are proteins that span the cell membrane

functional characteristics of ion channels

The currents that go through a single ion channel can be recorded, a flux of ions through a channel differs from diffusion, the opening and closing of a channel involves conformational changes.

What can ion channels be grouped as?

gene families.

what are the roles of ion transporters and pumps?

they are important for establishing and maintaining the concentration gradients of physiological important ions between thhe inside and outside of the cell.

What is the negative resting potential determined by?

a class of potassium channels that are 100-fold more permeable to potassium than sodium.

why do sodium ions flow into a cell through voltage gated sodium channels during action potential?

the external sodium concentration is much greater than the internal concentrations. the open channels will only allow sodium to diffuse down its concentration gradient.

What do pumps that maintain ion gradients use?

They use chemical energy in the form of ATP to transport ions against their electrical and chemical gradients (active transport)

why do ions attract water other than the fact that they are hydrophillic?

water molecules are dipolar. although the net charge on a water molecule is zero, the charge is separated within the molecules.

why does sodium move through the solution

it has a strong electrostatic attraction for water which causes it to have a large water shell.

the smaller the ion?

the lower its mobility in solution.

selective filter

an ion must shed most of its water of hydration to transverse the channel. In their place, weak chemical bonds form with polar (charged) amino acid resides that line the walls of the channel.

what is the structure of a phospholipid?

hydrophilic head and hydrophobic head where the hydrophobic tails join to exclude water and ions whereas the polar hydrophilic heads face the aqueous environments of the extracellular fluid and cytoplasm.

how is the kinetic properties of ion permeation best described by?

the channels conductance, which is determined by measuring the current (ion flux) through the open channel in response to an electrochemical driving force.

what is electrochemical driving force determined by?

the electrical potential difference across the membrane and the concentration gradients of the permeant ions across the membrane changing either one one can change the net driving force.

a the current of a nonlinear function

this type of channel behaves as a rectifier because it conducts ions more readily in one direction than in the other because of asymmetry in the channels structure or ionic environment

what would be the rate of the ion flux (current) through a channel if it was at low concentrations?

the current increases almost linearly with concentration.

what would be the rate of the ion flux (current) through a channel if it was at high concentration?

the current tends to reach a point at which it no longer increases. At this point the current is said to “saturate”.

gating

in ion channels that mediate electrical signaling, the channel protein has two or more conformational states that are relatively stable. This is the transition of a channel between the different states.

where do localized conformational changes occur?

in one region of the channel

where do generalized structural changes occurs?

along the length of the channel

ligand gating

the ligand-gated channel opens when a ligand binds a receptor site on the external surface of the channel protein. the energy from ligand binding drives the channel toward an open state.

phophorylation gating

some channels are regulated by protein phosphorylation and dephosphorylation. the energy for channel opening come from the transfer of the high-energy phosphate.

voltage gating

the voltage-gated channels open and close with changes in electrical potential differences across the membrane. the change in membrane potential causes a local conformational change by acting on a region of the channel that has a net charge.

stretch or pressure gating

some channels open and close in response to membrane stretch or pressure. the energy for gating may come from mechanical forces that are passed to the channel either directly by distortion of the membrane lipid bilayer or by protein filaments attached to the cytoskeleton or surrounding tissues.

change in membrane potential

many voltage-gated channels enter a refractory state after briefly opening in response to depolarization of membrane. they recover from the refractory state and return to the resting state only after the membrane potential is restored to its resting value.

calcium binding

some voltage-dependent calcium channels become inactivated when the internal calcium level increases following channel opening. the internal calcium binds to calmoduin (CaM), a specific regulatory protein associated with the channel.

when do ligand gated channels enter refractory states?

when their exposure to the agonist is prolonged (desensitization)

when do voltage gated channels enter a refractory state?

after the channels open ina process called inactivation which is when the channel is closed and can no longer be opened by positive voltages.

competitive antagonists

they interfere with normal gating by binding to the same site at which the endogenous agonist normally binds.

noncompetitive gating

drugs will have an inhibitory neurotransmitter which enhance the frequency of the channels opening to an inhibitory like GABA binding.

How do ion channels get constructed as hetero-oligomers?

through distinct subunits coming together, homo-oligomers from a single type of subunit get together, or from a single polypeptide chain organized into repeating motifs, which each motif functions as the equivalent of one subunit.

ligand-gated channel (ACh receptor)

it is formed with members of a large family of ligand-gated channels get together.

Gap-junction channel

it is formed from a pair of hemichannels, one each in the pre- and postsynaptic cell membranes, that join in the space between two cells.

Voltage-gated channel (Na+ channel)

it is formed from a single polypeptide chain that contains four homologus domains which as motifs 1-4, each with six membrane-spanning alpha-helixes (S1-S6).